optional argument breaks multicolumn

Question

If you are using a command to append a multiline to a tabular, it will stop working if the command has optional arguments. It will work without them:

\documentclass{standalone}
\newcommand{\mwe}[2][1]{ % remove [1]
    \multicolumn{2}{l}{text}
}
\begin{document}
    \begin{tabular}{ll}
        \mwe[1]{1} % [1]->{1}
    \end{tabular}
\end{document}  

(Minimal working example, works when changed according to comments)

Error:

Misplaced \omit

Answer 1

This is a known problem: \multicolumn has to be the first object in a cell (after expanding macros), but the command with optional argument places something before it.

Use xparse.

\documentclass{standalone}
\usepackage{xparse}

\NewExpandableDocumentCommand{\mwe}{O{1}m}{%
    \multicolumn{2}{l}{text}%
}

\begin{document}
    \begin{tabular}{ll}
        \mwe[1]{1} % [1]->{1}
    \end{tabular}
\end{document}

So long as the list of arguments ends with m (a mandatory argument), the command is fully expandable.